Documentation: driver-api: PM: Add cpuidle document

Replace the remaining documents under Documentation/cpuidle/
with one more complete governor and driver API document for cpuidle
under Documentation/driver-api/pm/.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>

2 files changed, 286 insertions, 3 deletions

diff --git a/Documentation/driver-api/pm/cpuidle.rst b/Documentation/driver-api/pm/cpuidle.rst
new file mode 100644
index 000000000000..5842ab621a58
--- /dev/null
+++ b/Documentation/driver-api/pm/cpuidle.rst
@@ -0,0 +1,282 @@
+.. |struct cpuidle_governor| replace:: :c:type:`struct cpuidle_governor <cpuidle_governor>`
+.. |struct cpuidle_device| replace:: :c:type:`struct cpuidle_device <cpuidle_device>`
+.. |struct cpuidle_driver| replace:: :c:type:`struct cpuidle_driver <cpuidle_driver>`
+.. |struct cpuidle_state| replace:: :c:type:`struct cpuidle_state <cpuidle_state>`
+
+========================
+CPU Idle Time Management
+========================
+
+::
+
+ Copyright (c) 2019 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
+
+
+CPU Idle Time Management Subsystem
+==================================
+
+Every time one of the logical CPUs in the system (the entities that appear to
+fetch and execute instructions: hardware threads, if present, or processor
+cores) is idle after an interrupt or equivalent wakeup event, which means that
+there are no tasks to run on it except for the special "idle" task associated
+with it, there is an opportunity to save energy for the processor that it
+belongs to.  That can be done by making the idle logical CPU stop fetching
+instructions from memory and putting some of the processor's functional units
+depended on by it into an idle state in which they will draw less power.
+
+However, there may be multiple different idle states that can be used in such a
+situation in principle, so it may be necessary to find the most suitable one
+(from the kernel perspective) and ask the processor to use (or "enter") that
+particular idle state.  That is the role of the CPU idle time management
+subsystem in the kernel, called ``CPUIdle``.
+
+The design of ``CPUIdle`` is modular and based on the code duplication avoidance
+principle, so the generic code that in principle need not depend on the hardware
+or platform design details in it is separate from the code that interacts with
+the hardware.  It generally is divided into three categories of functional
+units: *governors* responsible for selecting idle states to ask the processor
+to enter, *drivers* that pass the governors' decisions on to the hardware and
+the *core* providing a common framework for them.
+
+
+CPU Idle Time Governors
+=======================
+
+A CPU idle time (``CPUIdle``) governor is a bundle of policy code invoked when
+one of the logical CPUs in the system turns out to be idle.  Its role is to
+select an idle state to ask the processor to enter in order to save some energy.
+
+``CPUIdle`` governors are generic and each of them can be used on any hardware
+platform that the Linux kernel can run on.  For this reason, data structures
+operated on by them cannot depend on any hardware architecture or platform
+design details as well.
+
+The governor itself is represented by a |struct cpuidle_governor| object
+containing four callback pointers, :c:member:`enable`, :c:member:`disable`,
+:c:member:`select`, :c:member:`reflect`, a :c:member:`rating` field described
+below, and a name (string) used for identifying it.
+
+For the governor to be available at all, that object needs to be registered
+with the ``CPUIdle`` core by calling :c:func:`cpuidle_register_governor()` with
+a pointer to it passed as the argument.  If successful, that causes the core to
+add the governor to the global list of available governors and, if it is the
+only one in the list (that is, the list was empty before) or the value of its
+:c:member:`rating` field is greater than the value of that field for the
+governor currently in use, or the name of the new governor was passed to the
+kernel as the value of the ``cpuidle.governor=`` command line parameter, the new
+governor will be used from that point on (there can be only one ``CPUIdle``
+governor in use at a time).  Also, if ``cpuidle_sysfs_switch`` is passed to the
+kernel in the command line, user space can choose the ``CPUIdle`` governor to
+use at run time via ``sysfs``.
+
+Once registered, ``CPUIdle`` governors cannot be unregistered, so it is not
+practical to put them into loadable kernel modules.
+
+The interface between ``CPUIdle`` governors and the core consists of four
+callbacks:
+
+:c:member:`enable`
+	::
+
+	  int (*enable) (struct cpuidle_driver *drv, struct cpuidle_device *dev);
+
+	The role of this callback is to prepare the governor for handling the
+	(logical) CPU represented by the |struct cpuidle_device| object	pointed
+	to by the ``dev`` argument.  The |struct cpuidle_driver| object pointed
+	to by the ``drv`` argument represents the ``CPUIdle`` driver to be used
+	with that CPU (among other things, it should contain the list of
+	|struct cpuidle_state| objects representing idle states that the
+	processor holding the given CPU can be asked to enter).
+
+	It may fail, in which case it is expected to return a negative error
+	code, and that causes the kernel to run the architecture-specific
+	default code for idle CPUs on the CPU in question instead of ``CPUIdle``
+	until the ``->enable()`` governor callback is invoked for that CPU
+	again.
+
+:c:member:`disable`
+	::
+
+	  void (*disable) (struct cpuidle_driver *drv, struct cpuidle_device *dev);
+
+	Called to make the governor stop handling the (logical) CPU represented
+	by the |struct cpuidle_device| object pointed to by the ``dev``
+	argument.
+
+	It is expected to reverse any changes made by the ``->enable()``
+	callback when it was last invoked for the target CPU, free all memory
+	allocated by that callback and so on.
+
+:c:member:`select`
+	::
+
+	  int (*select) (struct cpuidle_driver *drv, struct cpuidle_device *dev,
+	                 bool *stop_tick);
+
+	Called to select an idle state for the processor holding the (logical)
+	CPU represented by the |struct cpuidle_device| object pointed to by the
+	``dev`` argument.
+
+	The list of idle states to take into consideration is represented by the
+	:c:member:`states` array of |struct cpuidle_state| objects held by the
+	|struct cpuidle_driver| object pointed to by the ``drv`` argument (which
+	represents the ``CPUIdle`` driver to be used with the CPU at hand).  The
+	value returned by this callback is interpreted as an index into that
+	array (unless it is a negative error code).
+
+	The ``stop_tick`` argument is used to indicate whether or not to stop
+	the scheduler tick before asking the processor to enter the selected
+	idle state.  When the ``bool`` variable pointed to by it (which is set
+	to ``true`` before invoking this callback) is cleared to ``false``, the
+	processor will be asked to enter the selected idle state without
+	stopping the scheduler tick on the given CPU (if the tick has been
+	stopped on that CPU already, however, it will not be restarted before
+	asking the processor to enter the idle state).
+
+	This callback is mandatory (i.e. the :c:member:`select` callback pointer
+	in |struct cpuidle_governor| must not be ``NULL`` for the registration
+	of the governor to succeed).
+
+:c:member:`reflect`
+	::
+
+	  void (*reflect) (struct cpuidle_device *dev, int index);
+
+	Called to allow the governor to evaluate the accuracy of the idle state
+	selection made by the ``->select()`` callback (when it was invoked last
+	time) and possibly use the result of that to improve the accuracy of
+	idle state selections in the future.
+
+In addition, ``CPUIdle`` governors are required to take power management
+quality of service (PM QoS) constraints on the processor wakeup latency into
+account when selecting idle states.  In order to obtain the current effective
+PM QoS wakeup latency constraint for a given CPU, a ``CPUIdle`` governor is
+expected to pass the number of the CPU to
+:c:func:`cpuidle_governor_latency_req()`.  Then, the governor's ``->select()``
+callback must not return the index of an indle state whose
+:c:member:`exit_latency` value is greater than the number returned by that
+function.
+
+
+CPU Idle Time Management Drivers
+================================
+
+CPU idle time management (``CPUIdle``) drivers provide an interface between the
+other parts of ``CPUIdle`` and the hardware.
+
+First of all, a ``CPUIdle`` driver has to populate the :c:member:`states` array
+of |struct cpuidle_state| objects included in the |struct cpuidle_driver| object
+representing it.  Going forward this array will represent the list of available
+idle states that the processor hardware can be asked to enter shared by all of
+the logical CPUs handled by the given driver.
+
+The entries in the :c:member:`states` array are expected to be sorted by the
+value of the :c:member:`target_residency` field in |struct cpuidle_state| in
+the ascending order (that is, index 0 should correspond to the idle state with
+the minimum value of :c:member:`target_residency`).  [Since the
+:c:member:`target_residency` value is expected to reflect the "depth" of the
+idle state represented by the |struct cpuidle_state| object holding it, this
+sorting order should be the same as the ascending sorting order by the idle
+state "depth".]
+
+Three fields in |struct cpuidle_state| are used by the existing ``CPUIdle``
+governors for computations related to idle state selection:
+
+:c:member:`target_residency`
+	Minimum time to spend in this idle state including the time needed to
+	enter it (which may be substantial) to save more energy than could
+	be saved by staying in a shallower idle state for the same amount of
+	time, in microseconds.
+
+:c:member:`exit_latency`
+	Maximum time it will take a CPU asking the processor to enter this idle
+	state to start executing the first instruction after a wakeup from it,
+	in microseconds.
+
+:c:member:`flags`
+	Flags representing idle state properties.  Currently, governors only use
+	the ``CPUIDLE_FLAG_POLLING`` flag which is set if the given object
+	does not represent a real idle state, but an interface to a software
+	"loop" that can be used in order to avoid asking the processor to enter
+	any idle state at all.  [There are other flags used by the ``CPUIdle``
+	core in special situations.]
+
+The :c:member:`enter` callback pointer in |struct cpuidle_state|, which must not
+be ``NULL``, points to the routine to execute in order to ask the processor to
+enter this particular idle state:
+
+::
+
+  void (*enter) (struct cpuidle_device *dev, struct cpuidle_driver *drv,
+                 int index);
+
+The first two arguments of it point to the |struct cpuidle_device| object
+representing the logical CPU running this callback and the
+|struct cpuidle_driver| object representing the driver itself, respectively,
+and the last one is an index of the |struct cpuidle_state| entry in the driver's
+:c:member:`states` array representing the idle state to ask the processor to
+enter.
+
+The analogous ``->enter_s2idle()`` callback in |struct cpuidle_state| is used
+only for implementing the suspend-to-idle system-wide power management feature.
+The difference between in and ``->enter()`` is that it must not re-enable
+interrupts at any point (even temporarily) or attempt to change the states of
+clock event devices, which the ``->enter()`` callback may do sometimes.
+
+Once the :c:member:`states` array has been populated, the number of valid
+entries in it has to be stored in the :c:member:`state_count` field of the
+|struct cpuidle_driver| object representing the driver.  Moreover, if any
+entries in the :c:member:`states` array represent "coupled" idle states (that
+is, idle states that can only be asked for if multiple related logical CPUs are
+idle), the :c:member:`safe_state_index` field in |struct cpuidle_driver| needs
+to be the index of an idle state that is not "coupled" (that is, one that can be
+asked for if only one logical CPU is idle).
+
+In addition to that, if the given ``CPUIdle`` driver is only going to handle a
+subset of logical CPUs in the system, the :c:member:`cpumask` field in its
+|struct cpuidle_driver| object must point to the set (mask) of CPUs that will be
+handled by it.
+
+A ``CPUIdle`` driver can only be used after it has been registered.  If there
+are no "coupled" idle state entries in the driver's :c:member:`states` array,
+that can be accomplished by passing the driver's |struct cpuidle_driver| object
+to :c:func:`cpuidle_register_driver()`.  Otherwise, :c:func:`cpuidle_register()`
+should be used for this purpose.
+
+However, it also is necessary to register |struct cpuidle_device| objects for
+all of the logical CPUs to be handled by the given ``CPUIdle`` driver with the
+help of :c:func:`cpuidle_register_device()` after the driver has been registered
+and :c:func:`cpuidle_register_driver()`, unlike :c:func:`cpuidle_register()`,
+does not do that automatically.  For this reason, the drivers that use
+:c:func:`cpuidle_register_driver()` to register themselves must also take care
+of registering the |struct cpuidle_device| objects as needed, so it is generally
+recommended to use :c:func:`cpuidle_register()` for ``CPUIdle`` driver
+registration in all cases.
+
+The registration of a |struct cpuidle_device| object causes the ``CPUIdle``
+``sysfs`` interface to be created and the governor's ``->enable()`` callback to
+be invoked for the logical CPU represented by it, so it must take place after
+registering the driver that will handle the CPU in question.
+
+``CPUIdle`` drivers and |struct cpuidle_device| objects can be unregistered
+when they are not necessary any more which allows some resources associated with
+them to be released.  Due to dependencies between them, all of the
+|struct cpuidle_device| objects representing CPUs handled by the given
+``CPUIdle`` driver must be unregistered, with the help of
+:c:func:`cpuidle_unregister_device()`, before calling
+:c:func:`cpuidle_unregister_driver()` to unregister the driver.  Alternatively,
+:c:func:`cpuidle_unregister()` can be called to unregister a ``CPUIdle`` driver
+along with all of the |struct cpuidle_device| objects representing CPUs handled
+by it.
+
+``CPUIdle`` drivers can respond to runtime system configuration changes that
+lead to modifications of the list of available processor idle states (which can
+happen, for example, when the system's power source is switched from AC to
+battery or the other way around).  Upon a notification of such a change,
+a ``CPUIdle`` driver is expected to call :c:func:`cpuidle_pause_and_lock()` to
+turn ``CPUIdle`` off temporarily and then :c:func:`cpuidle_disable_device()` for
+all of the |struct cpuidle_device| objects representing CPUs affected by that
+change.  Next, it can update its :c:member:`states` array in accordance with
+the new configuration of the system, call :c:func:`cpuidle_enable_device()` for
+all of the relevant |struct cpuidle_device| objects and invoke
+:c:func:`cpuidle_resume_and_unlock()` to allow ``CPUIdle`` to be used again.
diff --git a/Documentation/driver-api/pm/index.rst b/Documentation/driver-api/pm/index.rst
index 2f6d0e9cf6b7..56975c6bc789 100644
--- a/Documentation/driver-api/pm/index.rst
+++ b/Documentation/driver-api/pm/index.rst
@@ -1,9 +1,10 @@
-=======================
-Device Power Management
-=======================
+===============================
+CPU and Device Power Management
+===============================
 
 .. toctree::
 
+   cpuidle
    devices
    notifiers
    types